The proposed research involves a systematic study of the synthesis and properties of a class of mu-oxo mu-carboxylato Fe2 complexes containing biologically relevant polyimidazole ligands. These compounds will be used as synthetic analogues of the active sites on oxo-iron proteins such as: hemerythrin (Hr), ribonucleotide reductase (RR), purple acid phosphatase (PAP) and methane monooxygenase (MMO). These proteins are thought to have similar mu-oxo-mu-carboxylato Fe2 core structures, histidine as a ligand, stabilize Fe2II,FeIIFEIII and Fe2III oxidation levels, and perform very different biological functions (which may be dependent on the nature of ancillary ligands present at the active sites). Few studies to date have involved a systematic evaluation of the structure and properties of Fe2 complexes containing imidazole ligands. We have developed rational methods for preparing a wide range of polyimidazole ligand frameworks and will use these ligands to stabilize FE2II, FeIIFeIIIandFe2III complexes. A novel class of bis-tripodal and symmetric and asymmetric cyclic polyimidazole ligands will be prepared. These compounds will be used in anion (N3-,PO43- , CN-etc.) binding studies (as functional models of Hr and PAP). We will evaluate ligand strain effects on the structural and electronic properties of oxo-bridged Fe2 complexes in order to gain a better understanding of how imidazole functions as a ligand. The physical properties of all Fe2II, FeIIFeIII and Fe2III compounds will be assessed by a compliment of physical techniques, such as: X-ray crystallography, magnetic susceptibility, EPR, NMR, IR, UV-visible, Mossbauer spectroscopies as well as magnetic and electrochemical methods. Results from these studies will be compared with data on the proteins and other model complexes. We will evaluate the propensity of the Fe2III complex to function as alkane oxidation catalysts (model of MMO). Finally, we propose to extend our preliminary studies on the dynamic trapping/detrapping process observed in solid samples of a class of FeIIFeIII complexes of the phenol containing polyimidazole ligand Hbimp and explore the origin of the g<2 EPR signals observed for these complexes.